1. Field of the Invention
The present invention relates to an ultrasound transducer for the use in high intensity focused ultrasound (HIFU) applications for medical applications.
2. Description of the Background Art
Lysis (the process of disintegration or dissolution) of human tissue using high intensity focused ultrasound (HIFU) is a technique that has been studied for over 50 years. Research into applications of HIFU have revolved mostly around treating malignant tumors in the body either untreatable by other means, or promising a more efficacious treatment modality. HIFU commercialization has been very slow to develop, however, despite some of its early promise. Reasons for this include the inability to visualize the lesions being formed, the necessity of having to lyse an entire malignant tumor to be considered effective, and especially the extended period of time required to lyse a significant volume of tissue. In the last 10 years or so, some companies have been formed to commercialize HIFU for non-cancerous treatment applications. The best-known example is the treatment of enlarged prostate or BPH. Here, it is not necessary to lyse all the tissue to be effective. Advances in the treatment of BPH with drugs, however, has seriously shrunk the commercial prospects for this case. Advances in MRI and diagnostic ultrasound in the last 20 years have aided visualization of HIFU lesions, alleviating a main obstacle to the commercial advancement of the field. Additionally, where small volumes of tissue can be treated, as in hemostasis and blood clot breakup, HIFU is likely to prove to be viable commercially. Lysing large volumes of tissue, as in the case of removing significant amounts of adipose tissue, requires additional technical strategies.
Cross-sectional histological views of a HIFU lesion formed in vivo in porcine adipose tissue are shown at 40× (FIG. 1) and at 200× (FIG. 2). Tri-chrome staining is employed. Lysed tissue shows blood perfusion around individual fat cells and incursion of phagocytes and red blood cells into the HIFU-treated volume. Lesions formed with HIFU are typically cigar-shaped, with lesions lengths being 5–8 times the diameters. HIFU lesions are typically formed by spherically focusing an ultrasonic beam created by driving electrically a piezoelectric material, causing it to vibrate. These lesions can be as little as 1–2 mm in diameter, and 6–10 mm in length. It would take many lesions to necrose a large volume of tissue. Preferred thermal processes are generally slow, so that it may take up to 30 seconds to generate enough absorption of the ultrasonic energy to raise the temperature high enough to necrose tissue, and additional time to allow the temperature of tissue between the skin and treated volume to cool sufficiently before the next lesion is created. A simple calculation shows that it could take hours to ablate a volume of tissue on the order of 250 cc with a single transducer making individual lesions. Diminishing the time required to ablate a large volume of tissue, as in adipose tissue reduction, could mean the difference between a successful commercial product and an unsuccessful one.
Several strategies can be employed to reduce the time between making individual lesions, such as using multiple, mechanically scanned HIFU transducers, scanning the transducer(s) continuously, and/or using some kind of linear or 2D transducer array or structure to generate multiple focal spots. While some combination of these strategies could be employed, a major physical limitation of reducing scanning time remains the small diameter of the focused ultrasound beam. A defocusing strategy holds some promise of increasing the effective spot size by spreading the energy more laterally than lengthwise, creating a more spherical-shaped lesion. “Wobbling” the HIFU transducer mechanically about its axis could serve to do this, and such ideas have been reported in the literature. However, it would be certainly less complicated and expensive to build the defocusing into the HIFU transducer itself.
A method of creating an annular focal zone where the diameter of the annulus is adjustable has been reported by Cain and Umemura (Cain, Charles A. and Shin-Ichiro Umemura, “Concentric-Ring and Sector-Vortex Phased-Array Applicators for Ultrasound Hyperthermia”, IEEE Trans. on Microwave Theory and Techniques, Vol MTT-34, No. 5, May 1986, pp. 542–551.) and (Umemura, Shin-Ichiro and C. A. Cain, “The Sector-Vortex Phased Array: Acoustic Field Synthesis for Hyperthermia”, IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, Vo. 36, No. 2, March 1989, pp. 249–257.) and Hynynen et. al. (Fjield, T., V. Sorrentino, H. Cline, and K. Hynynen, “Design and experimental verification of thin acoustic lenses for the coagulation of large tissue volumes”, Phys. Med Biol., Vol 42, 1997, pp. 2341–2354.) and (Fjield, T. and K. Hynynen, “Experimental Verification of the Sectored Annular Phased Array for MRI Guided Ultrasound Surgery”, Proc 1996 IEEE Ultrasonics Symposium, pp. 1273–1276.) and (Fjield, T. and K. Hynynen, “The Combined Concentric-Ring and Sector-Vortex Array for MRI Guided Ultrasound Surgery”, IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, Vo. 44, No. 5, September 1997, pp. 1157–1167.) and (Fjield, T., N. McDannold, C. Silcox, and K. Hynynen, “In Vivo Verification of the Acoustic Model Used to Predict Temperature Elevations for MRI Guided Ultrasound Surgery”, Proc 1998 IEEE Ultrasonics Symposium, pp. 1415–1418.). This concept, dubbed the sector-vortex array, has been implemented using electronic array techniques or simply adding a mechanical sector-vortex lens onto the front of a planar transducer. The effect of this lens is to create a double-cone field pattern that yields an annular ring in cross-section.
Extensive research with mechanically scanned, spherically focused transducers has been conducted in HIFU fat-tissue mimicking gel phantoms and in vivo porcine adipose tissue. The purpose of this research is to determine basic design parameters for optimizing the lysing of subcutaneous adipose tissue and develop candidate lysing and scanning protocols for inclusion into a product development specification and initial human safety trials. This research has shown that challenges remain in reducing the overall treatment time to the desired values. While various scanning strategies have proven fruitful in reducing the heat build-up tissue in the zone between the skin and HIFU-created focal volume, and thus reducing scanning time, even greater efficacy can be shown if the focal zone overall diameter is increased significantly. Transducers with built-in vortex capability could prove to he a relatively simple, cost-effective means to achieve this capability.
Spherical focusing is typically achieved by bonding a plano-concave lens on to the front of a planar piezoelectric material, or shaping the piezoelectric material into a spherical bowl. High ultrasonic intensities (1–4 kW/cm2) are created driving the piezoelectric material at very high power levels, and focusing the beam very tightly. Absorption of this energy by tissue elevates its temperature. Raising this temperature in excess of 60° C. in the focal zone coagulates cell proteins, thereby ablating the tissue. There is a very sharp line between ablated and unablated tissue, as seen in FIG. 1. If the intensity is high enough, the temperature rise is sufficient to cause boiling and the production of small bubbles, producing a lesion with the elongated shape of a weather balloon at high altitudes (“tadpole” shape to others). At an even higher threshold, inertial cavitation (bubble creation) may occur, which can lyse tissue through thermo-mechanical means.
Lesions made in vivo in porcine tissue with spherically focused transducers show remarkably similar characteristics to those made in adipose tissue. Cigar-shaped lesions are made at relatively low power levels at longer insonification times, following the 6 dB contour of a classically spherically focused lens, and elongated weather balloon-shaped lesions are made at relatively high power levels where suspected boiling creates bubbles which reflect acoustic power back to the skin surface. At very high power levels, or too long insonification times, excessive heat can be generated above the focal zone which will lyse tissue up to and including the skin surface. While large volumes of tissue can be lysed in this manner, controlling this type of lesion growth can be problematic when considering patient and environmental variability.
Reducing the focal intensity at the center of the lesion and spreading the energy out over a larger volume should help alleviate the creation of a boiling hot spot while allowing more energy to be deposited during an insonification period, ultimately reducing the scanning time.
Composite piezoelectric materials have been a subject of research and development for nearly 25 years. Ultrasonic transducers and arrays for diagnostic imaging purposes have been manufactured since the late 1980's, and have application in such diverse areas as sonar and non-destructive testing. One manufacturer (Imasonic, Besancon, FR) builds custom composite HIFU applicators which are used by several research and commercial organizations. Popularly known as piezocomposites, these materials are made by taking solid blocks of piezoelectric ceramic, dicing the block into a forest of tall, thin pillars of ceramic, and backfilling the dicing kerfs with a polymer. This composite ceramic/polymer material is then processed like a normal solid transducer ceramic into thin plates. The volume fraction of the composite is controlled by the dicing kerf width and spacing, and the composite material properties can thereby be tailored to specific applications.
While many superior properties of composites are exploited in diagnostic imaging, the chief interest for HIFU is the ability to form composites into arbitrary shapes. In diagnostic imaging, the superior piezoelectric and acoustic properties of composites are exploited to produce wideband frequency responses, which are of little interest in HIFU at this time. Flexible polymer materials are often used as composite fillers which allow the material to be easily manipulated into cylindrical or spherical shapes. However, flexible composites are inherently high loss materials and unsuitable to the high power levels used in HIFU. Flexible materials would distort and ultimately disintegrate due to the heat build-up inside the material itself under high drive levels. Using less flexible polymers as composite filler materials would increase the composites' power handling capabilities due to lower intrinsic loss mechanisms, but would then make forming the composite into a curved shape seemingly impossible.
The answer to this dilemma is to make use of an interesting property of many hardset epoxies: these materials can be heated at a relatively low temperature into a partially cured state (namely, a B-stage cure) that is quite hard, but fairly brittle. In this state, the material can be processed, that is diced, filled, ground, lapped, and electroded into the thin sheets needed for HIFU applicators, and then reheated to a temperature somewhat above the original B-stage cure temperature. At this point, the polymer softens considerably and can be clamped into a mold shaped to the configuration desired (for instance, a spherically shaped bowl or vortex shape) and reheated to a much higher temperature. The epoxy filler then will fully cure, and further heat treatment can elevate the epoxy glass transition temperature, the temperature at which the polymer suddenly will soften, to levels between 120° and 200° C. The material thus formed is relatively low loss and capable of handling HIFU power levels, albeit with less efficiency than solid, high power, piezoelectric ceramics.
While requiring custom molds and clamping equipment, ultimately it is easier and cheaper to produce ceramic elements this way than to grind them directly using expensive equipment or hand labor; the elements are more rugged as well, which is a very important consideration. The lower efficiency is not expected to be a limitation in fat lysis since higher frequencies can be used for the small treatment depths, and thus high intensities can be achieved with relatively low drive levels. This is easily demonstrated by considering the fact that the intensity antenna gain for a focused radiator increases as the square of the frequency. An optimum frequency for HIFU at a given depth of focus can be obtained from the following equation:fopt=1/(2αz),where fopt is the optimum frequency, α is the ultrasonic absorption of fat, and z is the tissue depth to be treated. For fat treated at 2 cm of depth, the optimum frequency for maximum heat deposition is 4.2 MHz. Other considerations may change the actual operating frequency, of course.
Clearly the marriage of a technique of increasing the focal area of a HIFU applicator like the vortex concept and a means of easily and cheaply implementing this interesting structure like piezocomposites can reduce the overall treatment time for lysing large volumes of adipose tissue. Reducing treatment time and cost are key areas which will determine the fate of lysis commercialization efforts.
Sector-vortex array design has been extensively described and simulated by Cain and Umemura, Umemura and Cain, and numerous papers from Hynynen and co-workers at Brigham's and Women's Hospital at Harvard. They describe an implementation whereby the driving electrode of a spherical radiator is divided into N number of equal sized, pie-shaped sectors. Following Cain, driving signals are applied to the N sectors with a phase distribution over the N sectors determined by:φl=m(θl),
for l=1,2, . . . N where m is the vortex mode number, and θl=i2π/N. The phase distribution over the N sectors is such that that the excitation field rotates around the radiator at a phase velocity ω0/m. When N is large, an approximate analytic expression for the acoustic field in the focal plane can be derived; this field has a shape determined by the mth order Bessel function. The vortex-shaped field is zero along the central axis for m≠0, and has a diameter proportional to the vortex mode m. The field in cross-section resembles an annulus with side lobes at radii greater than the annulus.
However sector-vortex designs require complex electronics to drive the transducers in order to produce the vortex focal field. The electronics are required to drive either sector transducers or phased arrays in sequence. Some prior teachings rely on ancillary technologies such as the use of MRI machines to detect hotspots, and provide for additional electronic to provide real time corrections in the electronic firing of the various transducer elements to eliminate or reduce the occurrence of out-of-field ultrasound excitation. Complex lenses have also been described to facilitate the steering of the ultrasound energy into the body.
Thus there remains a need in the art for a robust vortex transducer, having a simplified design that can operate without the requirement of complex and expensive electronics.
There is further a need for a vortex transducer that can be aimed without the use of a lens.
There is still further a need for a vortex transducer having a fast activation and treatment time for reliably depositing a fixed amount of energy into a focal zone.